1. Field of the Invention
The present invention relates to a transverse electromagnetic (TEM) mode resonator which serves as an essential component of a small-size high-frequency filter to be employed in a wireless device such as a car telephone or a satellite communication device or in a voltage controlled oscillator (VCO).
2. Description of the Prior Art
In recent years, with a growing demand for movable wireless devices, in particular for car telephones, there has been a remarkable technical development in the art. In the technical development of the wireless devices, miniaturization of high-frequency filters has especially been in a rapid progress.
Each conventional high-frequency filter was constructed by air cavity type TEM mode resonators in its early production stage. However, after discovery of a ceramic material having a perovskite structure with high dielectric constant and low loss characteristics, each resonator was filled with such ceramic material thereby to reduce the length of the resonator to 1/er.sup.(1/2) (er: relative dielectric constant). With the reduction of the length of the resonator mentioned above, the diameter of the resonator was also reduced. Currently, a material having a relative dielectric constant (er) of not smaller than 90 has been developed to reduce the length of the resonator to approximately one tenth of that of the air cavity type and the improvement thereof has been used as a component of a portable compact telephone and the like.
There lies a problem in the production process of the above-mentioned new type filter which makes it necessary to process a frequency adjustment therefor. A ceramic material is obtained through a firing process, and the physical constants thereof depend on the conditions of the firing process, small variations in mixture ratio of impurities, and other factors. Therefore, any filter constructed by combining resonators employing the above-mentioned ceramic material indispensably necessitates a frequency adjustment procedure. Conventionally, there has been known as an example of frequency adjustment means a coaxial dielectric resonator loaded with a trimmer having a frequency adjustment function.
The following describes the coaxial dielectric resonator (TEM mode resonator) with reference to the attached drawings.
FIG. 14 shows an exploded view of a conventional TEM mode resonator having a frequency adjustment function. Referring to FIG. 14, reference numeral 200 denotes a thick and large cylindrical dielectric body, where the circumferential surface thereof is covered with an outer conductor 205 and the bottom surface thereof is covered with a bottom conductor 206. The inner peripheral wall of the cylindrical body is also covered with an inner conductor serving as a central conductor 211 which is connected to the bottom conductor 206 at the bottom surface of the dielectric body 200. On an open end surface 201 of the dielectric body 200, there is provided an electrode 204 serving as a trimmer fixing electrode in connection with the outer conductor 205 as shown in FIG. 14. On the other hand, a rotor dielectric plate 209 is provided with a rotor electrode 221 disposed on a half area of the upper surface thereof. The coaxial dielectric body 200 and the rotor plate 209 constructed as mentioned above are combined with each other by inserting a conductor shaft 207 therethrough and secured with a ring member 210 at the top.
The central conductor 211 and the conductor shaft 207 have the same potential, while the rotor electrode 221 connected to the conductor shaft 207 has the same potential as that at a tip portion of the central conductor 211. There is provided a ring conductor 202 coaxially disposed on the open end surface 201 for stabilizing position of the rotor so as to ensure smooth rotation of the rotor.
The following describes the operation of the TEM mode resonator constructed as mentioned above.
One end surface of the coaxial portion is short-circuited via the bottom conductor 206, while at the other end surface, the central conductor and the outer conductor 205 are connected via a trimmer capacitor having a capacitance Ct with a trimmer function, and the coaxial portion has its length shorter than one-fourth of the wavelength of the resonance frequency. In the above case, the resonance frequency of the resonator having the coaxial structure can be expressed by an equation as follows. EQU Zo.multidot.tan (.beta.h)=1/(.omega.Ct)
where Zo represents a characteristic impedance, .beta. represents a propagation constant, h represents the length of the coaxial portion, and .omega. represents an angular frequency. The resonance frequency varies depending on the variation of the capacitance Ct.
As another example for varying the resonance frequency, there is a method of varying the length of the resonator. A TEM mode coaxial dielectric resonator having its one end short-circuited and the other end opened has a resonance frequency where the electric length of the resonator is one forth of the resonance frequency. The most effective means for changing the length of the resonator is to mechanically abrade the open end surface, which method is currently used widely.
On the other hand, the same frequency adjustment is necessary for a TEM mode resonator to be employed in a VCO. By connecting a varactor diode having a capacitance Cv in parallel with a resonator having trimmer construction as mentioned above, the resonance frequency can be varied with a voltage variably applied to the varactor diode, thereby varying the oscillation frequency. Since each varactor diode has a different capacitance, the center frequency can be adjusted by adjusting the trimmer capacitance Ct. When employing a resonator having no trimmer construction, there is also adopted a method of connecting a varactor diode in parallel with the coaxial resonator in the same manner as described above, where the center frequency is controlled by abrading the tip end surface of the coaxial resonator.
However, the coaxial dielectric resonator having the trimmer construction has several drawbacks because the resonator is bulky due to its inherent construction. That is, in view of the growing demand for further reducing the size of the current portable wireless device to a pocket size, the resonator size is required to be as small as possible. Furthermore, in order to secure a sufficient trimmer capacitance, there is a certain limitation in reducing the area of the open end surface of the coaxial body and consequently in reducing the diameter of the coaxial resonator, which is also an obstacle in reducing the size of the resonator. For the same reason, when such a coaxial resonator is employed in a VCO, it is necessary to prevent the variation in capacitance of the varactor diode from exceeding the compensation range with the trimmer, and therefore, the trimmer size and the resonator diameter cannot be reduced.
On the other hand, the method of abrading the open end surface in the axial direction of the coaxial body is very simple, however, the most serious drawback is that the end surface cannot be restored after it is abraded. For restoration, it is necessary to connect somewhat capacitive component or attach a dielectric chip so that the electric field at the open end concentrates. Either one of the above-mentioned restoration procedures requires a troublesome work resulting in significantly reducing the productivity. Particularly when producing a filter having a plurality of resonators to be adjusted at the same time, even skilled workers are required to perform a time-consuming processing work with high concentration and care so as not to excessively abrade the end surface.